The invention relates to a series of at least two electro-magnetic deflection units for colour display tubes of the in-line type having the same deflection angles and neck diameters but at least two different screen formats, in which each deflection unit is provided with:
a first deflection coil having a front end and a rear end for deflecting electron beams generated in the display tube in a vertical direction, the electron beams, when the deflection unit has been mounted on a display tube, passing through the coil in the direction from the rear end towards the front end, as well as PA1 a second deflection coil, which coil is of the saddle type and also has a front end and a rear end, for deflecting electron beams generated in the display tube in a horizontal direction, a yoke ring of ferromagnetic material being provided around the at least the second deflection coil. PA1 a first deflection coil of the saddle type having a front end and a rear end for deflecting electron beams generated in the display tube in a vertical direction, the electron beams, when the deflection unit has been mounted on a display tube, passing through the coil in the direction from the rear end towards the front end: PA1 a second deflection coil of the saddle type also having a front end and a rear end for deflecting electron beams generated in the display tube in a horizontal direction, as well as a yoke ring of ferromagnetic material surrounding the two deflection coils, which series is characterized in that the first and second deflection coils at their front ends have a cup-shaped portion which is adapted to the outer surface of the display tube, and at their rear ends have a cylindrical portion which is adapted to the surface of the display tube on the one hand the dimensions and the shape of the cup-shaped portion of the first deflection coils and on the other hand the shape and the dimensions of the cup-shaped portion of the second deflection coils in display tubes of different screen formats being the same, the effective length of the cylindrical portion of the first coil increasing and that of the second deflection coil decreasing when the screen format of the display tube for which they are mounted increases, and conversely. (An example of a series of display tubes is, for example, a series having a constant deflection angle of 110.degree. and 20, 22 and 26 inch screens).
For some time a colour display tube has become the vogue in which three electron beams are used in one plane; the type of such a cathode ray tube is sometimes referred to as "in-line." In this case, for decreasing convergence errors of the electron beams, a deflection unit is used having a line deflection coil which, for deflecting the electron beams in a horizontal direction, generates a pin-cushion field, and a field deflection coil which, for deflecting the electron beams in a vertical direction, generates a barrel-shaped field. Within the scope of the invention, such a deflection unit may comprise in particular the combination of a field deflection coil of the so-called saddle shell type with a line deflection coil of the so-called saddle shell type. A coil of the saddle type is to be understood to mean herein a coil which is constructed from two coil halves, the front and rear ends of each coil half extending approximately perpendicularly to the plane in which the electron beams lie, and a coil of the saddle shell type is to be understood to means herein a coil which is constructed from two coil halves in which the front end of each coil half extends approximately perpendicularly to the plane in which the electron beams lie, and the - cylindrical - rear end is adapted to the outer surface of the neck part of the display tube.
Deflection units for in-line colour display tube systems can in principle be made to be entirely self-convergent, that is to say in a design of the deflection unit which insures convergence of the three electron beams on the axes, anisotropic y-astigmatism errors, if any, can simultaneously be made zero in the corners without this requiring extra correction means. Where it would be interesting from a point of view of manufacture to have a deflection unit which is self-converging for a series of display tubes of the same deflection angles and neck diameters but different screen formats, the problem exists, however, that a deflection unit of given main dimensions can be used only for display tubes of one screen format. This means that only one screen format can be found for a fixed maximum deflection angle in which a given deflection unit is self-converging without a compromise (for example, the use of extra correction means).
It is the object of the invention to provide a method of the kind mentioned in the opening paragraph with which it is possible, starting from deflection coils having given main dimensions, to compose self-converging deflection units for a series of display tubes of different screen formats.
Within the scope of the invention this object is achieved in that for a given screen format the first and the second coil each have a given effective length between their front and rear ends, the effective length of the first coil being larger and for the effective length of the second coil being smaller for a larger screen format, and conversely, so as to provide for different screen formats a self-converging combination of display tube/deflection unit.
The invention is based on the recognition of the fact that, if self-convergence on the axes has been reached, the possibly remaining anisotropic y-astigmatism error (the so-called y-convergence error in the corners) mainly depends on the distance between the line deflection point and the field deflection point and to a much smaller extent on the main dimensions and the shape of the deflection coils used. Now if deflection units for different screen formats are to be composed while using deflection coils having the same shape and main dimensions, the distance between the line and field deflection points may be used as a parameter to nevertheless achieve self-convergence for a family of display tubes having different screen formats but the same maximum deflection angles.
Within the scope of the invention, the variation in the distance between the line and field deflection points necessary for adapting to different screen formats is achieved by increasing or decreasing the effective coil length of either the line coil or the field coil, or of both but then in the opposite sense, with the main dimensions of the deflection coils remaining the same and with the dimensions of the yoke ring remaining the same, for example, by mechanically making the coil or coils on the rear side smaller and longer, respectively, by a few millimeters, or by positioning, with the coil length remaining the same, the window farther or less far to the rear (so that the turns on the rear side are more or less compressed). As will be explained hereinafter, all this can be carried out very simply in practice when saddle-shaped coil halves of the shell type are used at least for the line coil and preferably also for the field coil.
The invention actually involves that, for use of a deflection unit in a display tube having a larger screen format than the display tube for which it is designed, the deflection points of the line deflection field and field deflection field generated by the given deflection unit must be moved apart and, for use in a display tube having a smaller screen format, they must be moved towards each other.
The use of the invention results, in particular, in a series of at least two combinations display tube/deflection unit, the display tubes having the same neck diameters and deflection angles but different screen formats, each deflector unit comprising:
As will be described in greater detail hereinafter with reference to the method of the invention, the great advantage of the invention is that for adaptation to the various screen formats of a given series, only a very small alteration in the length of the (cylindrical) rear section of the individual deflection coils is necessary to obtain the desired variation in the distance between the deflection points. This means that the complicated cup-shaped portion may remain unvaried as regards dimensions so that self-converging deflection coils for display tubes of different screen formats can be made by means of one jig (having an adjustable rear section). In order to maintain convergence on the axes, the wire distribution in the cup-shaped portion of the coils needs at most only small alterations and in fact this applies only to the line coil. The main geometry, however, remains unchanged.
The invention therefore also relates to a method of assembling electromagnetic deflection units for colour display tubes of the in-line type having the same deflection angles and neck diameters but at least two different screen formats in which a first deflection coil of the saddle type having a front end and a rear end, a cup-shaped portion at the front end and a cylindrical portion at the rear end, for deflecting electron beams generated in the display tube in a vertical direction, the electron beams, when the deflection unit has been mounted on a display tube, passing through the coil in the direction from the rear end towards the front end, is combined with a second deflection coil, which coil is of the saddle type and has a front end and a rear end, a cup-shaped portion at its front end and a cylindrical portion at its rear end, for deflecting electron beams generated in the display tube in a horizontal direction, a yoke ring of ferromagnetic material being provided around the assembly of the two deflection coils, characterized in that at least the second deflection coil is composed of two identical halves which are wound on a jig having a cup-shaped portion and a cylindrical portion. The shape and the dimensions of the cup-shaped portion being the same for each screen format, the cylindrical portion of the jig, however, having an adjustable body for determining the length of the cylindrical portion of the coil halves.
A variation .DELTA.D in the distance between line and field deflection point is produced by varying the effective length of the line coil with respect to that of the field coil. .DELTA.D is linearly associated with the variation of the screen format, in which the relation applies that: EQU .DELTA.D=.beta..DELTA.Z.sub.s,
where .DELTA.Z.sub.s is the variation in the distance from the front end of the coil situated nearest to the screen (this generally is the line coil) to the screen. The value of .beta. is roughly between 0.05 and 0.15.